1. Field of the Invention
The present invention relates to a transfer apparatus and an image forming apparatus which form images by an electrophotographic method using developing agent transferred onto a transfer material (for example, a paper) with a transfer belt.
2. Description of Related Art
In recent years, demand has increased to form a full-color image as well as a monochromatic image by an image forming apparatus of electrophotographic type, and such an electrophotographic full-color image forming apparatus is under development. Normally, the full-color image forming apparatus forms images using color toner (developing agent) corresponding to each image data of a plurality of colors decomposed from a color image. For example, the same color image is read through each of the filters of each color (red, green, blue) of the three primary colors for additive color mixture, and an image data of each color (cyan, magenta, yellow) of at least the three primary colors for subtractive color mixture is created from the read data. Based on the image data of each color, a visible image is generated using toner of the corresponding color, and these visible images of the respective colors are superposed one on another thereby to form a full-color image.
In this full-color image forming apparatus, the exposure process, the development process and the transfer process are required for each color, while at the same time occurring the problem of aligning the visible images of the respective colors in position. In view of this situation, the rate at which the full-color image is formed is apparently considered lower than the rate at which the monochromatic image is formed. To overcome this problem, a full-color image forming apparatus of tandem type has conventionally been proposed in which a plurality of image forming units for forming visible images of different colors are arranged in line on the outer peripheral surface of a rotatable semiconductive endless belt along the direction of movement thereof, so that a full color image may be formed before the endless belt makes at least one rotation.
To increase the speed of forming a full-color image, the full-color image forming apparatus of tandem type employs an intermediate transfer method in which the visible images of the respective colors formed in the image forming units are superposed one on another on the outer peripheral surface of the endless belt and then transferred onto the paper, or a transfer conveyance method in which the visible images of the respective colors formed by the image forming units are transferred sequentially onto the surface of a transfer material (for example, a paper) conveyed by adsorption on the outer peripheral surface of the endless belt (for example, Japanese Patent Application Laid-Open No. 10-039651 (1998) and Japanese Patent Application Laid-Open No. 10-293437 (1998) and Japanese Patent No. 2574804).
FIG. 1 is a schematic diagram for explaining the configuration of the essential portion of the conventional full-color image forming apparatus employing the intermediate transfer method. The full-color image forming apparatus shown in FIG. 1 comprises an image forming unit 200 of electrophotographic type, in which a full-color image is formed on the paper through a primary transfer process for transferring the toner images of the respective colors in superposed relation with each other on a transfer belt 201 and a secondary transfer process for transferring onto the paper the multi-color toner image formed on the transfer belt 201 in the primary transfer process. The transfer belt 201 is configured to move along the direction of the white arrow by a transfer belt driving roller 202 and a transfer belt driven roller 203. Photosensitive drums 204a through 204d corresponding to the respective colors (for example, yellow, magenta, cyan and black) and intermediate transfer rollers 205a through 205d in opposed relation to the photosensitive drums 204a through 204d, respectively, are arranged along the path of the transfer belt 201.
In such conventional full-color image forming apparatus, consider a case in which an image is printed based on the image data inputted from the external. First, the electrically charged toner images of the respective colors are formed on the surface of the respective photosensitive drums 204a through 204d. Then, high-voltage transfer bias is applied to the intermediate transfer rollers 205a through 205d, so that the toner images on the photosensitive drums 204a through 204d are sequentially transferred onto the transfer belt 201. In the process, the transfer timing of the respective toner images is controlled, so that the toner images of the respective colors are superposed one on another and a single multi-color toner image is formed on the transfer belt 201. Then, a high voltage of opposite polarity to the charge polarity of the toner is applied to the transfer roller 206 arranged in the subsequent stage of the primary transfer process, with the result that a multi-color toner image is transferred on the paper supplied from a paper feeding unit 210. The paper onto which the multi-color toner image has been transferred is conveyed to a fixing unit not shown, where the multi-color toner image is fixed on the paper. Thus, a printed matter formed with a full-color image is completed.
In the image forming apparatus described above, the transfer belt 201 carrying the toner image in the primary transfer process is held by the respective photosensitive drums 204a through 204d and the intermediate transfer rollers 205a through 205d, and the toner image on the photosensitive drums 204a through 204d is transferred onto the transfer belt 201 under a comparatively high pressure. As a result, with the progress of the transfer process for the respective colors, the adhesion between the toner and the transfer belt 201 is increased to such an extent that the cohesion between the toners is promoted. Thus, the resolution is reduced, and the transfer failure is liable to occur at the time of electrostatic transfer in the secondary transfer process.
In view of this, the present inventors earlier proposed an image forming apparatus (Japanese Patent Application No. 2004-43342) in which the intermediate transfer rollers 205a through 205d are arranged in spaced relation downstream side of the transfer process from the positions opposed to the photosensitive drums 204a through 204d in such a manner as to reduce the contact pressure between the transfer belt 201 and the photosensitive drums 204a through 204d. In such image forming apparatus, the increase in adhesion between the toner and the transfer belt 201 can be reduced while at the same time preventing the cohesion between the toners. Thus, the toner image can be satisfactorily transferred and a satisfactory image quality obtained in the secondary transfer process.
In the transfer system of the above mentioned image forming apparatus newly proposed by the present inventors, however, the intermediate transfer rollers and the photosensitive drums are arranged in spaced relation with each other, and a current or a voltage is applied through the transfer belt interposed between them. The toner transfer, therefore, is affected by the action corresponding to the current change characteristic or the voltage change characteristic of the transfer belt. Therefore, unlike in the prior art, the transfer performance of the primary transfer process cannot be determined simply by the guideline of the resistance value of the transfer belt, and it has become apparent that a stable toner transfer may not be conducted even with the transfer belt having the same resistance value. In order to improve the image quality, therefore, a guideline is required to select a transfer belt material applicable to the transfer system newly proposed by the present inventors, and the transfer system is required to be configured using the transfer belt formed in accordance with the guideline.